From 5d87d8307f460a16167416a0e808bcb1c74295b4 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Thu, 14 Dec 2023 09:50:47 +0100
Subject: [PATCH] interpret: extend comment on the inhabitedness check in
 downcast

---
 .../src/interpret/projection.rs                | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/compiler/rustc_const_eval/src/interpret/projection.rs b/compiler/rustc_const_eval/src/interpret/projection.rs
index 0f3b6b25c61..9a034ba22b9 100644
--- a/compiler/rustc_const_eval/src/interpret/projection.rs
+++ b/compiler/rustc_const_eval/src/interpret/projection.rs
@@ -208,6 +208,24 @@ pub fn project_downcast<P: Projectable<'tcx, M::Provenance>>(
         if layout.abi.is_uninhabited() {
             // `read_discriminant` should have excluded uninhabited variants... but ConstProp calls
             // us on dead code.
+            // In the future we might want to allow this to permit code like this:
+            // (this is a Rust/MIR pseudocode mix)
+            // ```
+            // enum Option2 {
+            //   Some(i32, !),
+            //   None,
+            // }
+            //
+            // fn panic() -> ! { panic!() }
+            //
+            // let x: Option2;
+            // x.Some.0 = 42;
+            // x.Some.1 = panic();
+            // SetDiscriminant(x, Some);
+            // ```
+            // However, for now we don't generate such MIR, and this check here *has* found real
+            // bugs (see https://github.com/rust-lang/rust/issues/115145), so we will keep rejecting
+            // it.
             throw_inval!(ConstPropNonsense)
         }
         // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.